refactor: replace unwrap with idiomatic get_or_insert_with in add_submodule

This change refactors `SubmoduleEntries::add_submodule` to use
`get_or_insert_with(HashMap::new)` instead of a manual `is_some()` check
followed by `unwrap()`. This improves code readability and follows Rust
idioms for handling `Option` types.

Co-authored-by: bashandbone <89049923+bashandbone@users.noreply.github.com>

Author: google-labs-jules[bot]

src/config.rs

@@ -739,22 +739,10 @@ impl SubmoduleEntries {
     }
 
     /// Add a submodule entry
-    #[must_use] pub fn add_submodule(self, name: SubmoduleName, entry: SubmoduleEntry) -> Self {
-        if self.submodules().is_some() {
-            let mut submodules = self.submodules.unwrap();
-            submodules.insert(name, entry);
-            Self {
-                submodules: Some(submodules),
-                sparse_checkouts: self.sparse_checkouts,
-            }
-        } else {
-            let mut submodules = HashMap::new();
-            submodules.insert(name, entry);
-            Self {
-                submodules: Some(submodules),
-                sparse_checkouts: self.sparse_checkouts,
-            }
-        }
+    #[must_use] pub fn add_submodule(mut self, name: SubmoduleName, entry: SubmoduleEntry) -> Self {
+        let submodules = self.submodules.get_or_insert_with(HashMap::new);
+        submodules.insert(name, entry);
+        self
     }
 
     /// Remove a submodule entry